The present invention concerns a doctor beam for a doctor blade for application in a printing unit, which doctor beam has a front side with a U-shaped channel constituting a part of the doctor blade chamber. The invention furthermore concerns a doctor blade for application in a printing unit and comprising a doctor beam with a front side with a U-shaped channel constituting a part of the doctor blade chamber.
Doctor blades are well-known for use in rotary flexographic printing, a printing method being particularly widespread within the packing industry. By flexographic printing, ink is transferred to paper, cardboard, plastic, metal film or similar print carrier by means of a rubber plate having a balanced amount of ink. The ink is transferred by means of a raster roller that today normally is a ceramically coated metal roller which has a multitude of tiny holes or cells, typically 10–100 μm deep, in its surface.
By varying the number of holes and hole depths it is possible to vary the amount of ink transferred and which is typically 3–25 g/m2. In order to ensure that the holes are only filled to the rim, a doctor blade is scraping across the roller. This doctor blade is most often mounted in a closed ink supply system comprising a doctor blade. This is an elongate, closed chamber formed by the U-shaped channel, where at each side there is mounted doctor blades being in contact with the raster roller which forms one side of the chamber. At its ends, the chamber is closed by end walls or packings.
The doctor beam is usually made of metal, preferably aluminium, due to the mechanical properties desired in connection with chambers which may have lengths of one meter or more. Alternatively, doctor beams have also been made of plastic materials. These doctor blades however, have limited application due to the mechanical properties of the material.
Today, basic inks are used, thus causing the problem with corrosion of beams of aluminium. In order to relieve this, coating the beams with TEFLON, or at least their front side being in contact with aggressive inks, has been attempted. However, this is unfavourable as TEFLON is only partly pH-resistant. Alternatively, metal may be coated by nickel-plating or chromium-plating. This is, however, difficult if not impossible to do, particularly when the beam is of aluminium.
From DE-197 25 056 is known a doctor blade where a coating is provided on surfaces in order to achieve reduced friction. No solution to the problem with corrosion is indicated.
In spite of the widespread use of doctor blades, until now there has been no possibility for providing doctor beams with good mechanical properties and which at the same time are resistant against the chemical action from the applied inks, particularly for aluminium beams.